The aim of the proposed project is to obtain basic information on the site and mechanism of synthesis of dipalmitoylphosphatidylcholine-the major surface-active component of pulmonay surfactant - both in the adult rabbit lung and during fetal development. In particular the role of the alveolar type II cell in the synthesis of phosphatidylcholine, especially by deacylation and reacylation of performed phosphatidylcholine will be established. The role of hormones, such as corticosteroids, thyrotropin-releasing hormone (TRH) and estrogens, on fetal lung maturation and surfactant production will be assessed. In particular the effect of estrogens on the synthesis of phosphatidylcholine will be examined in detail. The effect of estradiol-administration to pregnant rabbits on the activities of a number of enzymes involved in phospholipid synthesis in the fetal lung will be examined. The effect of TRH on secretion of surfactant into the alveoli will be determined. Mixtures of these hormones will be given simultaneously in order to determine if their effects are additive. These studies are of clinical importance since immaturity of the lung with insufficient surfactant is believed to be the cause of respiratory distress syndrome of the newborn and information on lung maturation and acceleration is essential for its prevention and/or control.